Helium occurs naturally in very low concentrations within underground natural gas and carbon dioxide reservoirs. In some instances, helium is present at sufficiently high concentrations to justify its recovery. In general, helium can be effectively recovered from gas streams containing at least 0.1 mole percent helium or greater. Helium is typically first concentrated into a crude helium stream that contains about 70 mole percent helium. The crude helium can be stored, typically in underground reservoirs, or subsequently further purified and liquefied for merchant sale.
There is a growing demand for large quantities of carbon dioxide and applications such as enhanced oil recovery. This demand has increased the interest of extracting valuable helium from such carbon dioxide rich streams. It is to be noted that vast quantities of carbon dioxide are processed in enhanced oil recovery applications, normally greater than 30,000 tons per day. The high unit volume of helium can substantially improve overall project economics.
In the prior art, natural gas-hydrocarbon streams have been subjected to helium extraction and purification. For instance, in U.S. Pat. No. 3,355,902, a helium containing stream, that predominantly contains nitrogen and hydrocarbons and a minute quantity of carbon dioxide, is cooled and introduced into the fractionation column to separate the vapor from the liquid phases. The gaseous column overhead, that contains negligible amounts of carbon dioxide, is then passed into a phase separator to produce a gaseous stream enriched in helium and a liquid stream. The gaseous stream is further phase separated and the liquid is further fractionated to produce a crude helium stream and a liquid stream.
In U.S. Pat. No. 5,329,775, a cryogenic helium production system is disclosed for separating helium from a stream that contains helium, hydrocarbon and carbon dioxide. Again the helium and carbon dioxide is present within the feed in very low concentrations. The feed is rectified within a liquid vapor contact column to produce a tower overhead which is further cooled and phase separated to produce the helium containing stream.
Both of the patents, mentioned above, are not applicable to the recovery of helium from carbon dioxide containing streams obtained from known underground reservoirs in which carbon dioxide is present at concentration levels greater than 30 mole percent. The low temperature processes illustrated in these patents would be ineffective for high levels of carbon dioxide in the feed due to the fact that the carbon dioxide would solidify.
As will be discussed, the present invention provides a method for separating helium from a gaseous carbon dioxide containing feed that contains at least about 30 mole percent or higher. Such method allows the carbon dioxide containing fraction to be repressurized and returned for use or storage at high pressure. In case of feed streams having a high content of light components, such as nitrogen, the recompression can be carried out in an energy efficient manner given that the volume to be repressurized is lower than that of the feed. As to the separated light components, the present invention is intended to be used with further known purification techniques to produce a crude helium stream that can be stored or further processed (e.g. liquefied). As will be discussed, such method is also applicable to separating other light components such as hydrogen and neon from feed streams having similarly high carbon dioxide contents.